1. Field of the Invention
The present invention relates to a motor driving circuit.
2. Description of the Related Art
Some motor driving circuits may keep the amounts of currents flowing through motor coils at set levels by controlling on and off of transistors connected to the motor coils (Japanese Patent Application Laid-Open Publication No. 2005-184897). For example, in a motor driving circuit shown in FIG. 5, an energization state of a motor coil M connected between terminals T1 and T2 is controlled by on and off of N-channel MOSFETs 110 to 113 making up an H-bridge. For example, in a drive control circuit 120, the N-channel MOSFETs 110 and 113 are turned on and the N-channel MOSFETs 111 and 112 are turned off, so that the circuit passes the current through a path of the N-channel MOSFET 110, the motor coil M, and the N-channel MOSFET 113, to drive the motor (driving state). The amount of the current flowing through the motor coil M is detected by a resistor R connected via a terminal T3 and when the amount of the current flowing through the motor coil M reaches the set level, the drive control circuit 120 turns off the N-channel MOSFET 110 and turns on the N-channel MOSFET 111. As a result, the current, which the motor coil M tries to continue passing, is regenerated by a loop of the N-channel MOSFET 111, the motor coil M, and the N-channel MOSFET 113, and decreases gradually (regeneration state). As such, the drive control circuit 120 is capable of maintaining the amount of the current flowing through the motor coil M at the set level by repeating the driving state and the regeneration state.
Incidentally, in the motor driving circuit, a load may be short-circuited due to aged deterioration of the motor, for example. In the motor driving circuit shown in FIG. 5, if the load is short-circuited, an overcurrent occurs in a case where the energization state of the motor coil M is the driving state, and if the overcurrent is over the set level of the current flowing through the motor coil M, the drive control circuit 120 changes the energization state of the motor coil M to the regeneration state. Then, after elapse of a predetermined time, the drive control circuit 120 changes the energization state of the motor coil M to the driving state, and the driving state and the regeneration state are repeated despite the load is short-circuited. Since the motor driving circuit is generally provided with an overheat protection function, circuit operation stops before causing circuit failure even if the driving state and the regeneration state are repeated in a state where the load is short-circuited. It is desirable, however, to detect a short circuit of the load and protect the circuit before it becomes in such an overheated state.